Many of the active substances or auxiliary agents suitable for use in the manufacture of active substance patches or TTSs show a tendency to discolor, for example to yellow. Such adverse changes may also occur during the application period. It is known, for instance, that nicotine patches gradually turn yellow.
The aforementioned changes are inmost cases due to oxidative decomposition processes which progress upon contact with atmospheric oxygen and moisture especially during storage of the active substance patch or when it is being worn on the skin and which are promoted by action of light. Particularly affected by such processes are pharmaceutical active substances, antioxidants, various enhancers (i.e. substances promoting or accelerating transdermal active substance absorption), as well as oxidation-sensitive components of the pressure-sensitive adhesive that is present in the active substance patch, such as resin adhesives, for example.
The extent of active substance decomposition does not necessarily have an adverse effect on the pharmaceutical quality of the products, for instance if the resulting decomposition products amount to only fractions of a weight percent of the starting composition and if these decomposition products are toxicologically acceptable. Thus, discoloration often already affects a product cosmetically whereas the pharmaceutical quality is still unimpaired. Frequently, users or patients especially in the case of medicaments associate such disadvantageous changes in the optical appearance of the active substance patches with defectiveness or deterioration, which causes a feeling of insecurity in those patients.
Often these changes are yellow, brown or red discolorations as typically appear in chemical decomposition. Even slight changes in color may be interpreted by the users or patients as indicative of a deterioration of the quality of the medicament.
The problem of discoloration occurs particularly if the product, in fresh condition after manufacture, initially appears colorless or white to the human eye and the above-mentioned discoloration occurs only after a certain period of storage or while the patch is being worn on the skin. This is perceived by the users to be even more critical and potentially dangerous than a discoloration which has been there from the start and only becomes more intense during storage.
In the field of medical active substance patches, transparent and colorless patches represent the ideal case in respect of cosmetics since the user himself or other persons regard them as inconspicuous when applied to the user's skin. Users of medicinal patches generally prefer patches with such inconspicuous properties because they reduce the risk of other people becoming aware of the user's need for treatment and possibly finding out about his illness.
If for reasons of cosmetics a transparent design of an active substance patch does not make sense, for example because the ingredients are colored or because of discoloration occurring during storage, it is possible to equip the patch with a non-transparent backing layer. During the application period this backing layer then prevents that the color or discoloration is optically perceived.
In the latter case, it is disadvantageous, however, that patches or TTSs equipped with a nontransparent backing layer are much more conspicuous at the site of application, that is, on the patient's skin, than transparent or colorless patches. A measure known from the state of the art and frequently applied consists in applying a skin-colored lacquer to the nontransparent backing layer. This, however, leads to a further problem since it proves extremely difficult to find a skin tone that in equal measure suits a larger number of users of different skin color tone and is cosmetically acceptable. Taking into consideration all of the skin types of the world population, it is entirely impossible to determine a unitary, opaque skin color tone that would be suitable as the color tone for a nontransparent backing layer. This problem could be solved, it is true, by producing otherwise identical active substance patches having differently colored backing layers that match the different skin color tones of the world population, but this is out of the question because of the complex manufacturing and distribution logistics, and ultimately for reasons of cost.